This invention relates to prevention of losses of vanadium in the hydrogen sulfide scrubbing and oxidation process wherein a gaseous mixture is scrubbed with an aqueous alkaline scrubbing agent containing five-valent vanadium as the oxidizing agent. The hydrogen sulfide is oxidized to elementary sulfur while the five-valent vanadium is reduced to four-valent vanadium. The reduced scrubbing agent is subsequently regenerated for reuse by treatment with an oxygen-containing gas for the reoxidation of the four-valent vanadium.
Processes for the simultaneous absorption and oxidation of hydrogen sulfide are known (for example, Brennstoffchemie [Fuel Chemistry], Volume 50, 1969, No. 4, pages T24 and T25), in which the hydrogen sulfide is initially bound as hydrogen sulfide by the alkali, and then reacts with the oxidizing agent, a salt of anthraquinonedisulfonic acid and/or five-valent vanadium, with liberation of sulfur and reduction of the oxidizing agent. In order to reactivate the solution, air is blown through an oxidizer and, by this means, the reduced oxidizing agent is oxidized. At the same time, the sulfur floats to the surface of the solution and collects there as a supernatant scum which is drawn off, so that the sulfur, in turn, can be removed therefrom, for example, by filtration.
It has been demonstrated that, in the regeneration of the reduced scrubbing solution, either effective or real losses of vanadium can occur. Inasmuch as the solubility of four-valent vanadium is less than that of five-valent vanadium, on occasion the former may precipitate out and then be no longer available for the subsequent hydrogen sulfide scrubbing. Even with no precipitation, the reoxidation of the reduced vanadium often cannot be completely carried out because the expense is industrially unacceptable, and in this case only a part of the vanadium dissolved in the scrubbing agent is available in the five-valent form for the subsequent loading of the scrubbing agent with hydrogen sulfide, thereby creating an effective vanadium loss. Furthermore, these losses of vanadium must be compensated for by using concentrations of vanadium which are higher than those stoichiometrically necessary for the conventional concentrations of CO.sub.3.sup.-- ions of about 8 g/l of scrubbing agent.* There is, in turn, the danger that vanadium can thereby precipitate. This tendency of vanadium to precipitate has also been observed when the scrubbing agent has remained too long in the reduced state. FNT *(Here and in the following CO.sub.3.sup. -- concentration is the total alkalinity in the scrubbing solution expressed as carbonate.)